Check that you have SP3 files in C:\WINDOWS\ServicePackFiles\i386. The folder size would be between 200 and 300 MB.
If yes, you need to make a simple Registry change in Windows XP. You must have admin level access to do this.!Warning!
Editing the Windows' Registry may result in your computer stopping to work. Please create a backup of your Windows' Registry before you make any changes to it.
You can use ERUNT
to create a backup of your Windows' Registry.
Open Registry Editor and browse to the following location
Change the value to C:\WINDOWS\ServicePackFiles
Restart computer (or just log off and then log on again)
Try running SFC now.